1. Field of the Invention
The present invention relates to a nitride-based semiconductor device, and more particularly to a nitride-based semiconductor device for use in light emitting diodes, laser diodes, and the like, which has an improved active layer structure to achieve a reduction in operating voltage, so that it has an improved output efficiency.
2. Description of the Related Art
Generally, nitride semiconductors are widely used in green or blue light emitting diodes (LEDs) adapted as a light source for full-color displays, image scanners, various signal systems, and optical communication appliances. In such an LED, its active layer generates light in accordance with the principle of electron-hole recombination, and emits the generated light.
The active layer of such an LED may have a single quantum well (SQW) structure or a multiple quantum well (MQW) structure having a plurality of quantum well layers each having a thickness of less than about 100 Å. In particular, the MQW structure has been preferably used because an active layer having the MQW structure exhibits a superior optical efficiency-to-current ratio and a high emission power, over an active layer having the SQW structure.
FIG. 1a is a sectional view illustrating a conventional GaN-based semiconductor LED structure.
As shown in FIG. 1a, the GaN-based semiconductor LED denoted by the reference numeral 10 includes a sapphire substrate 11, a first nitride semiconductor layer 13 made of an n type GaN, an active layer 15 having an MQW structure, and a second nitride semiconductor layer 17 made of a p type AlGaN or p type GaN. An n type electrode 19a is formed on a mesa-etched upper surface of the second nitride semiconductor layer 17. Also, a transparent electrode layer 18 and a p type electrode 19b are sequentially formed on the first nitride semiconductor layer 13.
The active layer 15 having an MQW structure includes undoped GaN barrier layers 15a and undoped InGaN quantum well layers 15b alternately laminated over one another. FIG. 1b shows an energy band gap distribution of the MQW structure of the active layer 15. In FIG. 1b, the energy band gap is designated by “Eg”. Referring to FIG. 1b, it can be seen that the active layer 15 has a plurality of InGaN quantum well layers each interposed between GaN barrier layers having a large band gap. The active layer having such an MQW structure emits light by use of its quantum well layers arranged in series. Accordingly, the LED 10 can exhibit a superior optical efficiency-to-current ratio and a high emission power, over an those having the SQW structure.
However, the active layer 15 having an MQW structure inevitably has a relatively large thickness, as compared to active layers having an SQW structure because it has a multi-layer structure. For this reason, in the case having the above mentioned MQW structure, there may be problems of an increase in longitudinal serial resistance caused by an increased layer thickness, and thus, an increase in forward voltage (Vf).
In order to solve the above mentioned problems, a scheme has been proposed in which an n type impurity such as Si is doped into the quantum barrier layers. FIGS. 2a and 2b are energy band diagrams respectively illustrating active layer structures altered to achieve an improvement in forward voltage.
The structure of FIG. 2a may be a part of the semiconductor LED structure shown in FIG. 1. FIG. 2a shows an active layer 25 arranged between an n type GaN semiconductor layer 23 and a p type AlGaN semiconductor layer 27 while including four quantum well layers 25b and five quantum barrier layers 25a′ having a band gap larger than that of the quantum well layers 25b. This scheme utilizes the principle of doping an n type impurity in the quantum barrier layers 25a′ to reduce a resistance generated in the quantum barrier layers 25a when a voltage is applied across the semiconductor LED, thereby enhancing the probability of electron-hole recombination. Through this scheme, therefore, it is possible to produce a reduction in the forward voltage Vf.
Another structure similar to the above mentioned structure is illustrated in FIG. 2b. The scheme illustrated in FIG. 2b is adapted to improve the structure of FIG. 2a. This scheme is disclosed in the Korean Patent Laid-open Publication No. 2002-21121.
As shown in FIG. 2b, the LED structure according to this scheme includes an active layer 35 arranged between an n type GaN semiconductor layer 33 and a p type AlGaN semiconductor layer 37 while including four quantum well layers 35b and five quantum barrier layers 35a and 35a′ having a band gap larger than that of the quantum well layers 35b, similar to the structure of FIG. 2a. In this case, an n type impurity is doped in only a part of the five quantum barrier layers, that is, three quantum barrier layers 35a′. The three quantum barrier layers 35a′ doped with the n type impurity are arranged adjacent to the n type GaN semiconductor layer 33 while having a higher impurity concentration at a more adjacent one thereof to the n type GaN semiconductor layer 33. The reason why such a selective doping method is used is that the probability of electron-hole recombination at the side of the n type semiconductor layer 33 is lower than that at the side of the p type AlGaN semiconductor layer 37 because the mobility of holes is lower than that of electrons.
Thus, the scheme of selectively doping an n type impurity in the quantum barrier layers 35a′ exhibiting a low probability of electron-hole recombination may be used in order to obtain a superior optical efficiency-to-current ratio and a high emission power, in place of the scheme of achieving a reduction in the forward voltage Vf in accordance with doping of an n type impurity.
However, the above mentioned conventional schemes may cause a degradation in light emission efficiency because the n type impurity doped in the quantum barrier layers may be diffused into the quantum well layers. In the Japanese Journal of Applied Physics Vol. 37, 1998, pp. L431-L434, it is also reported that the growth mode of a quantum well layer of InGaN to be grown over a quantum barrier layer of GaN may vary in accordance with a variation in the amount of Si doped in the quantum barrier layer, so that the surface morphology of the quantum well layer may vary, thereby causing a degradation in optical characteristics. Thus, the scheme of doping an n type impurity in quantum barrier layers may practically adversely affect the light emission efficiency and optical characteristics of the resultant LED structure.
Therefore, a new scheme has been demanded in the technical field to which the present invention pertains, in order to reduce the resistance of quantum barrier layers in a nitride-based semiconductor device such as an LED, thereby achieving an improvement in forward voltage characteristics without causing a degradation in the optical characteristics of the active layer and a variation in process conditions.